1. Field of the Invention
This invention generally relates to a switch assembly and, more particularly, to a digital switch assembly suited for use as an input device for inputting numerical information such as, for example, temperature, time or count, to an electronic instrument, for example, a timer, a counter or a measuring device.
2. Description of the Prior Art
A digital input device currently widely used in, for example, a measuring instrument, comprises, as shown in FIG. 1 of the accompanying drawings, a digital switch unit 2 comprised of a plurality of switch assemblies 1 connected together in side-by-side relation to each other, which is in turn mounted on a common instrument panel 4 of, for example, a timer device 3. Each of the switch assemblies 1 forming the digital switch unit 2 has a pair of operating members 1a adapted to be manipulated by an operator one at a time to establish a particular electric circuit associated with a digital data to be inputed.
Where each of the switch assemblies SW is manufactured in a miniature size as it is a recent trend in the art, it has been found that, when one of the operating members in each switch assembly is depressed by the application of an external finger pressure, the finger of the operator is brought in the close vicinity of an internal circuit element, for example, a printed circuit board having a pattern of conductors formed thereon, which is located inside the switch assembly. By way of example, the finger tip of the operator depressing the operating member comes to a position spaced a distance of about 2 to 3 mm from the printed circuit board. Once the finger tip is brought to such a position, discharge of an electrostatic charge set up in the body of the operator takes place between the finger tip and a portion of the printed circuit board closest thereto, generating a high voltage which would adversely affect the circuit arrangement to such an extent as to result in erroneous operation of the circuit arrangement and/or damages to electronic component parts employed in the circuit arrangement.